Last year I was told there was a rule that only 3 people, plus mentor, were allowed in the pit area to work on your robot. I cannot find these in the rules this year, and was wondering if this is still the case this year?
I don’t believe that this was ever a rule officially in the FRC documents. It may have been one that was put in place by your team. I know many teams have far more than 3 people in the pit at a time, so it would be best to discus this with your fellow team mates 
3 total people are allowed in early on Thursday to uncrate the robot. That is the only limit FIRST puts on people coming in or out of the pits, besides that everyone must wear safety glasses.
A lot of individual teams have limits on the number of people allowed in their pit areas because of overcrowding. There is very little room and a lot of people reduce not only your team’s ability to work, but also other teams when you can’t all fit and spill out into aisles and other team’s pits. We try to keep somewhere in the 5ish range.
FIRST does not restrict the number of people in the pits. The first time i went to the Championship (back when it was still called nats in 2001) teams had there entire teams in the pits, sprawled out across the floor. A lot of teams have stopped doing it, but to answer your question FIRST does not restrict the number of people you can have in the pits working on your robot, but it is really up to whoever is in charge of your pits as to how many people should be in there.
It seems like I read somewhere they recommend, or encourage, reducing unnecessary people in pits just for safety concerns, but I don’t know where I read that.
Either way, they’re all right. No actual restricted limit on number of people in pits during the event.
The limit I believe is just for people allowed to do repairs on the robot between matches of finals. You can have 3 students and a mentor then.
There is no limit to the number of people allowed in the pits. As stated above, there is a limit to the number of people allowed to uncrate the robot on Thursday of competition. If there is a limit of people it is a team organized thing and not a FIRST rule. I would suggest having a pit crew of 3 students and a mentor, then 2 people that stand there and talk to people about your robot. Marketing is a big thing; you don’t want to forget to tell people what you can do because you might not get the chance in a match to do it. But I would like to say that please don’t have your entire team in the pit area. There isn’t enough room there for every team to have everyone in the pits. As a courtesy thing, I would keep the total below 10 people, and that’s allot of people in a 10’x10’ area.